1. Field of the Invention
The invention relates to laser devices useful in optical communications systems, particularly wavelength division multiplexing systems.
2. Discussion of the Related Art
As the use of optical communications continues to increase, the techniques for wavelength generation, selection, and maintenance have become more important. This is particularly the case for wavelength division multiplexing (WDM), in which precise and stable alignment of the source wavelength to a channel of the WDM system is necessary. However, because the emission wavelength of diode lasers tends to vary in response to temperature changes, various measures have been developed in an effort to stabilize emission of the desired source wavelength. One such measure is use of a fiber Bragg grating coupled to a semiconductor laser, where the laser is operated only a gain medium and the grating constitutes one reflective facet of the laser. This device is therefore typically referred to as an external cavity laser. The grating generally reflects only a selected wavelength such that the device lases only at the wavelength. Such an apparatus makes it possible to better ensure that the desired wavelength is emitted.
However, even these Bragg grating devices encounter a variety of stability issues, including thermal stability problems such as a mismatch between the thermal response of the diode versus the Bragg grating. These can significantly interfere with the operation of the laser, particularly where single mode output is desired. See, for example, U.S. Pat. No. 5,870,417 to Verdiell et al. (at Col. 2, lines 20-36). In response to these stability problems, Verdiell et al. present numerous—but complex—techniques that attempt to avoid or at least compensate for factors that lead to instability in the output wavelength of laser diode/grating devices, e.g., mode hopping. Simpler, and more commercially feasible, techniques would be preferred.
A separate problem in optical communications is coupling a semiconductor device (e.g., a diode) to a communications fiber—this coupling is difficult and problematic. For example, a very small displacement of the fiber relative to the semiconductor device output can lead to loss of more than half the light directed at the fiber. For this reason, coupling is typically performed by providing coupling optics between the fiber and the device. These optics can take many forms, including a tapered or conical lens formed or spliced onto the fiber, or a variety of other lens configurations. (Again, see Verdiell et al., supra, at Col. 4, lines 23-54.) Such coupling optics, however, add more complexity, both to the device as well as the overall packaging scheme. And, more significantly, even with these optics, precise alignment is still required.
Thus, improved techniques for overcoming these problems are desired.